Vessels are regularly tested for leak-tightness in many different technical fields. In the chemical processing industry, for example, testing is performed on liquid or gas vessels, or in the automotive industry, on tank systems.
In the automotive engineering industry in the United States, for example, future regulatory requirements for the operation of internal combustion engines will be more stringent. Accordingly, for motor vehicles in which volatile fuels such as gasoline are used, a control device may be included that may detect, by onboard means, the presence of any leak having a size of 0.5 mm, in the tank or in the entire fuel tank system.
Such a method for leak testing of a tank venting system of a motor vehicle is described in U.S. Pat. No. 5,349,935, German Published Patent Application No. 196 36 431, German Published Patent Application No. 198 09 384, and German Published Patent Application No. 196 25 702. As described in the referenced documents, the tank venting system is subjected to a positive pressure, and a conclusion as to the presence of a leak may be drawn from subsequent evaluation of the variation of pressure.
Similar methods are also described in Japanese Patent No. 6-173837 and U.S. Pat. No. 5,347,971 in which a reference leak is connected to the tank venting system and, by comparing measurements with and without the reference leak, a conclusion is drawn as to the presence of a leak.
In addition, German Published Patent Application No. 196 36 431 describes a method whereby a dynamic pressure is generated between an electrically driven pump and a reference leak having a cross section of 0.5 mm which pressure decreases the pump rotational speed and simultaneously increases its electrical power consumption. The intensity of the resulting steady-state electrical current is determined and temporarily stored, and the generated air flow of the pump is then pumped into the tank via a reversing valve, past the reference leak. If the tank is leak-tight, a higher pressure develops than when pumping is performed against the reference leak. The electrical power consumption of the pump is thus higher than in the case of the reference leak. For a leak having a cross-sectional opening larger than 0.5 mm, however, the pressure which develops is less than the reference pressure, and the power consumption is therefore lower.
Furthermore, German Published Patent Application No. 100 18 441 describes leak testing according to the aforementioned reference measurement principle; however, the method is performed by introducing a partial vacuum into the tank venting system.
Consequently, in the methods and devices described above, the presence of a leak is indicated only indirectly, based on the aforementioned power consumption of the pump. This has the disadvantage that the result of the leak diagnosis is greatly dependent on the characteristics of the pump used, such as the effect of moisture on the electrical pump current. The measured current then no longer corresponds to the existing pressure conditions, resulting in faulty identification of the degree of leak-tightness.